1. Field Of The Invention
This invention relates to a new apparatus used as a bubble forming and stabilizing device in a continuous extrusion process for making a blown film and a process for using same. Blown films may be made from any one of several processes and one such process is commonly referred to as a blown continuous extrusion process. The invention discloses an internal air deflector and bubble forming and stabilizing mandrel for use in internally cooling and stabilizing a bubble of blown film during the extrusion process. The device allows for increased production rates, improved stabilization and improved physical properties of the forming bubble by effectively forming the bubble over an internal mandrel enabling a high velocity cooling air stream to be directed between the under and outer surfaces of the mandrel and the inner surface of the forming bubble usually formed of a polymer. The invention enables more efficient heat transfer from the extrusion polymer to the cooling air stream causing the molten polymer to drop in temperature more quickly in the blowing process which subsequently also improves the stability of the process and further allows even higher internal and external air velocities to be introduced which in turn allows for increased productivity and improved product quality due to improved stability. The device also provides support for the molten polymer during its most unstable state.
2. Description Of The Prior Art
The device of the present invention is particularly suitable for use in a continuous process for the production of blown film. In many cases, the blown film will be formed from a polymer resin although other materials may be used to produce a blown bubble. For ease of reference, and not for limitation purpose, the following description will be made with reference to a bubble formdd from a polymer. In a typical process, a hot polymer melt is fed to a die from which it is extruded in the form of a tube which is nipped at a desired point after cooling to form a bubble. The extruded polymer is generally expanded by using internal air pressure to blow the polymer into a bubble and the bubble should be of uniform and constant thickness subsequent to the frost line. However, the tube which emerges from the die itself is generally unstable due to low melt strength until its temperature is reduced sufficiently to improve the melt strength and eventually solidify the polymer, that is, at its frost line
To increase the rate at which the molten bubble reaches the point of solidifying at the frost line, the temperature of the forming bubble is reduced as quickly as possible while still maintaining the desired stability. This may be done in several ways. One of several known methods is by using an external air ring which directs cooling air onto the outer surface of the forming bubble as it emerges from the die. Additional cooling can also be achieved by cooling the inside of the bubble such as is disclosed in U.S. Pat. No. 4,236,884 granted on Dec. 2, 1980 to Gloucester Engineering Co., Inc. The amount of cooling is generally limited by the temperature of the cooling air, the melt strength of the extrusion polymer, the blow-up ratio of the bubble size to the die size and the volume and velocity of cooling air that can be introduced to the inner and outer surfaces of the forming bubble without destroying the stability of the forming bubble. These limitations directly affect the production line speed and the product quality through the extrusion process.
Various devices have been proposed which attempt to reduce the temperature of the air within the forming bubble to improve the extrusion rate which in turn reduces production costs.
Cooling of the forming bubble can be achieved by cooling from the inside of the forming bubble or by outside cooling of the bubble, or by both. An example of the exterior cooling is shown in U.S. Pat. No. 4,259,947 granted to Robert J. Cole, the inventor herein. In this patent, there is disclosed a dual lip air ring wherein the exterior air is blown radially outwardly away from the forming bubble emerging from the die. The resulting venturi effect and low pressure zone causes the forming bubble to draw away from the medial line as it emerges from the die and allows a non-impinging, relatively high velocity air stream to be introduced to the exterior wall of the forming bubble, cooling it faster than direct impingement cooling. By cooling the forming bubble faste while maintaining the stability of the bubble, it is possible to increase the rate of extrusion of the bubble and maintain good quality thus reducing production time and costs.
Additional cooling can also be achieved from the inside of the bubble. As shown in U.S. Pat. No. 4,236,884, there is proposed a device which exchanges the hot interior air within the forming bubble with cooler air via ports located within the die mandrel itself. Air is supplied to a series of internal nozzles which blow the air radially outwardly at the internal surface of the forming bubble.
These and other processes of the prior art have clear limitations due to the effect of the impingement of the air and the low melt strength of the polymer during the blowing process. Further, as the formin bubble itself is increased in size with relation to the die size, the radial distance between the internal air nozzles and the wall of the forming bubble will also increase which has the undesired effect of reducing the efficiency of the cooling process.